Gear sets, or gear trains, are common to all mechanical or electro-mechanical systems requiring rotational motion control and power transmission, from simple machines such as wrist watches or wind-up toys to more complex systems such as automotive transmissions. A gear set consists of several mechanical gear elements, with each gear element engageable with at least one other gear element to transmit power and motion from one rotating body to another. Each gear element rotates or cycles at a cycle rate that may vary substantially from the cycle rates of other interconnected gear elements in the same gear set. The specific gear surface hardness chosen for any given application will depend in large part on the dynamics of the individual gear element and the forces or loads to which the individual gear elements are subjected.
Due to relatively high operating speeds and heavy loading, automotive power transmission gear sets are typically constructed from materials substantially harder than those used in lower speed, lower load gear sets. Gear sets used in automotive transmission systems in particular commonly include planetary gear sets comprised of sun gears, ring gears, and pinion gears.
A typical planetary gear set contains at least one high-cycle gear, for instance a sun gear, engaged or enmeshed with one or more lower-cycle gears such as a plurality of pinion gears. The sun gear typically sees more cycles than the pinion gear. The greater number of stress cycles or revolutions endured by the higher-cycle sun gear element may result in gradual, accumulative damage to the gear surfaces due to plastic deformation and friction, and in particular to those gear surfaces directly engaged with a mating gear element. Damage to the mating gear surfaces may take the form of abrasive wear, pitting, fatigue cracking, or other friction and stress-related deformation. Because of the relatively extreme operating conditions an automotive transmission may be subjected to, mating gear elements within such a system are typically hardened in an attempt to minimize damage inflicted on the gear elements during run-in and subsequent use.
It is known to use case hardening of gear elements in order to strengthen the overall gear set. Common case hardening methods include nitriding, carburizing, or carbon-nitriding processes by which nitrogen or carbon, respectively, are added to the external layers of the gear element to produce the desired surface hardness. In the use of case hardened gear elements, particularly in such gear element variations as are employed in automotive transmissions and other automotive gear systems, the surface hardness of the mating gears is matched as closely as possible within the tolerances of the gear elements, with the overall goal of minimizing damage to the surfaces of mating gears. As higher-cycle gear elements may experience increased levels of fatigue, stress cracking, and pitting relative to the lower-cycle mating gears, the higher-cycle gear may become the weakest member of the gear set, and therefore the primary source of costly premature gear set failure.
It is also known to apply surface coatings or spray coatings as a “hard” layer on mating metal parts. Common hard coatings include chromium, carbide, and titanium compounds. Fatigue performance of applied coatings is a noted drawback of hard coatings, due to the inherent risk of chipping, flaking, or the gradual reduction in bonding strength between the various material layers. Further, hard coatings are relatively expensive to apply, and repeatability of the applications within a narrow tolerance can be difficult and costly. In automotive planetary gear sets, applied hard coatings are commonly reserved for specialty applications, due in large part to these disadvantages.
It is also known to employ a hardness differential, i.e. a variance in surface hardness, between mating “soft” gear elements of a surface hardness of up to 35 on the Rockwell 30N (HR-30N) superficial hardness scale, in certain circular low speed/low stress gear sets. In such soft gear sets, the hardness differential is mainly used to compensate for manufacturing errors, such as errors in central distance and gear geometry, as changes in the shape or profile of the softer member occur through abrasive wear and/or plastic deformation. Such self-forming soft gear applications are typically limited to low speed drives employing circular gears in which the soft mating gears are not subject to the extreme stress, deformation, and accumulative wear of high cycle gears of the planetary gear type used in automotive transmissions.